1. Field of the Invention
The present invention generally relates to the art of electrochemical cells and, more particularly, to a lithium-containing cell system preferably comprising a primary lithium electrochemical cell electrically connected in parallel to a secondary electrochemical cell. A most preferred couple comprises a lithium oxyhalide cell electrically connected in parallel with a lithium ion cell.
2. Prior Art
Primary lithium oxyhalide cells are used extensively in applications requiring high gravimetric and volumetric energy density. Among the many sizes and chemistries available, cells can be developed for low rate or high rate applications and to operate from temperatures as low as −70° C. to as high as 200° C. The anode material usually consists of lithium or lithium alloyed with various elements such as aluminum, magnesium or boron and the cathode usually consists of some form of carbon which is held together using a suitable binder. The electrolyte generally consists of a solvent system of thionyl chloride, phosphoryl chloride or sulfuryl chloride. Often, additional compounds or interhalogen compounds such as sulfur dioxide, chlorine, bromine, bromine chloride and others may be dissolved therein to modify the cell for a particular purpose, such as extending the operating rate or temperature of the cell.
Electrolyte salts are also added to the solvent system to assist in ionic transfer during cell discharge. Such salts may include lithium chloride in combination with aluminum trichloride or gallium trichloride. Lithium tetrachloroaluminate salt (LAC) or lithium tetrachlorogallate salt (LGC) is then formed in-situ. Typically used catholytes include chlorinated sulfuryl chloride (CSC) having either LAC or LGC dissolved therein. These systems are commonly referred to as LAC/CSC and LGC/CSC. Another commonly used catholyte is thionyl chloride (SOCl2) having either LAC or LGC dissolved therein. Electrochemical systems based on this catholyte include LAC/SOCl2 and LGC/SOCl2. According to the present invention, the most preferred catholyte is thionyl chloride-bromine chloride (BCX) having either LAC or LGC dissolved therein.
While lithium oxyhalide cells are well known for their high energy and power density, there are some drawbacks to their use in particular applications. Unlike other pulse dischargeable lithium primary cells containing solid cathode systems and organic-based electrolytes, such as a lithium/silver vanadium oxide cell (Li/SVO) or a lithium/manganese dioxide cell (Li/MnO2), lithium oxyhalide cells have inferior rate capability. Additionally, when lithium oxyhalide cells are used after prolonged storage, their rate capability can be further limited by passivation. Passivation build-up at the anode/electrolyte surface layer is directly related to increased voltage delay when the cell is called upon to deliver a high power pulse.
According to the present invention, this relatively inferior rate capability characteristic of a lithium oxyhalide cell can be overcome to a great extent by electrically connecting the oxyhalide cell in parallel with a lithium ion, rechargeable cell. Importantly, the open circuit voltage of the lithium ion cell in a fully charged state is from about 0.05 volts to about 0.8 volts less than the open circuit voltage of the freshly built first primary cell. The thusly configured hybrid power source has both a high specific energy and a much improved capability for delivering high power pulses, even after long periods of storage.
U.S. Pat. No. 5,998,052 to Yamin describes some of the aspects of such a hybrid power source, but is specifically limited to a system in which “the open circuit voltage of said primary electrochemical cell is lower than the open circuit voltage of said rechargeable cell when said rechargeable cell is not connected to said primary cell and is fully charged.” Because the rechargeable cell in this prior art system is only partially charged, the specific energy of the overall system is lower than is desirable.
Therefore, the present invention improves upon the prior art hybrid power system by providing a primary cell electrically connected in parallel to a rechargeable cell. Importantly, the open circuit voltage of the fully charged lithium ion cell is equal to or less than the open circuit voltage of the primary cell. However, the difference is not so great as to cause failure or discharge capacity degradation of the secondary cell. The preferred primary cell is of lithium oxyhalide chemistries.